The charger measures how much "juice" it puts into the battery each session, not the current capacity of the battery.
If you have a 3000mAH battery and if it is 40% full, after fully charging the rest 60%, the UMS2 will display 3000 x 60% = 1800mAH.
If the same battery is 90% full when it is put into the charger, after fully charging the rest 10%, the charger will display 3000 x10% = 300 mAH.
If you want to have a rough estimate of the battery capacity, you have to fully discharge the battery and check how much mAH it takes to charge it back to full. However, we do not recommend doing this. A lithium-ion battery usually stops discharging once its output voltage drops to below a certain level. Attempting to over-discharge it will likely damage and kill the battery.